One approach for elucidating protein-protein binding in cells is the yeast-based two-hybrid system (Fields and Song (1989) Nature 340:245). That system utilizes chimeric genes and detects protein-protein interactions via the activation of reporter-gene expression. Reporter-gene expression occurs as a result of reconstitution of a functional transcription factor caused by the association of fusion proteins encoded by the chimeric genes. Typically, polynucleotides encoding two-hybrid proteins are constructed and introduced into a yeast host cell. The first hybrid protein consists of the yeast Gal4 DNA-binding domain fused to a polypeptide sequence of a known protein (often referred to as the "bait"). The second hybrid protein consists of the Gal4 activation domain fused to a polypeptide sequence of a second protein (often referred to as the "prey"). Binding between the two-hybrid proteins reconstitutes the Gal4 DNA-binding domain with the Gal4 activation domain, which leads to the transcriptional activation of a reporter gene (e.g., lacZ or HIS3), which is operably linked to a Gal4 binding site.